Para-xylene is an important raw material in the chemical and fiber industries. For example, terephthalic acid derived from para-xylene is used to produce polyester fabrics. Para-xylene is usually separated from a mixture of para-xylene and at least one other C8 aromatic hydrocarbon by either crystallization, adsorptive separation, or a combination of these two techniques.
U.S. Pat. No. 5,012,038 discloses that para-diethylbenzene (p-DEB) has become a commercial standard as a desorbent for the separation of para-xylene from other xylene isomers, but that p-DEB suffers in the process for separating para-xylene from feed mixtures containing C9 aromatics because the boiling point of p-DEB is too close to the boiling points of C9 aromatics in the feed. This reference also discloses that because C9 aromatics are difficult to separate from p-DEB by simple fractionation, the C9 aromatics would gradually build up in the desorbent which must be recycled for economic reasons; therefore, it has become necessary to reduce C9 aromatics in the feed to below about 0.1% prior to the adsorptive separation of para-xylenes.
R. A. Meyers, Handbook of Petroleum Refining Processes second edition McGraw-Hill Book Company (1997) in Chapter 2.6 discloses an adsorptive separation process for separating para-xylene from other xylene isomers. Feedstock specifications disclosed at page 2.48 of the chapter include maximum limits of 100 ppm for methylethylbenzenes and 500 ppm for other C9 aromatics.
U.S. Pat. No. 5,012,038; U.S. Pat. No. 4,886,930; U.S. Pat. No. 5,057,643; U.S. Pat. No. 5,171,922; U.S. Pat. No. 5,177,295; and U.S. Pat. No. 5,495,061 disclose the use of desorbents having higher boiling points than p-DEB to separate para-xylene from a feed mixture that contains C8 and C9 aromatics. The C9 aromatics are then separated from the higher boiling desorbent by fractionation. However, despite the benefits provided by the higher boiling adsorbents, p-DEB continues to be a commonly used desorbent for the adsorptive separation of para-xylene from C8 aromatic hydrocarbons. Thus, there remains a need in the art for improved processes that enable the separation or recovery of multiple components from C8 and C9 aromatic hydrocarbon mixtures.